1. Field of the Invention
The present invention relates to a semiconductor device having a circuit composed of a thin film transistor (hereinafter, TFT) and a method for manufacturing the semiconductor device. In particular, the present invention relates to an electronic device onboard an electric optical device typified by a liquid crystal display panel and a light emitting display device having an organic compound light emitting layer as components.
A semiconductor device in this specification means comprehensive semiconductor devices such as an electric optical device, a semiconductor circuit, and an electronic device.
2. Description of the Related Art
In recent years, study of a light emitting device having an EL element as a self-luminous element has become vigorous. In particular, a light emitting device using an organic material as an EL material has attracted an attention. The light emitting device is also referred to as an EL display.
Note that an EL element includes a layer containing an organic compound that emits light by applying an electric field (hereinafter, an EL layer), an anode, and a cathode. Luminescence generated by an organic compound is fluorescence that generates upon returning of electrons from the singlet excited state to the ground state and phosphorescence that generates upon returning of electrons from the triplet excited state to the ground state. A light emitting device fabricated by a deposition device and a deposition method is applicable to both kinds of luminescence.
A light emitting device has no viewing angle difficulties for its self-luminous property differently from a liquid display device. Thus the light emitting device is more suitable for using at outside than the liquid crystal display device. Various types of usage have been proposed for the light emitting device.
An EL element has a structure in which a pair of electrodes sandwich an EL layer between each other, generally, a laminated structure. Typically, a laminated structure, “a hole transporting layer, a light emitting layer, an electron transporting layer” proposed by Tang et al. of Kodak Eastman Company is generally known. The structure has greatly high luminous efficiency and employed by almost all light emitting devices that are under development now.
Another structure such as “an anode, a hole transporting layer, a light emitting layer, and an electron transporting layer” or “an anode, a hole injecting layer, a hole transporting layer, a light emitting layer, an electron transporting layer, and an electron injecting layer” can be also applicable. Fluorescent pigments can be doped to the light emitting layer. For forming these layers, either a low molecular material or a high molecular material can be used.
In this specification, an EL layer is a generic term used to refer to all layers formed between a cathode and an anode. Therefore all of each the above-mentioned hole injecting layer, hole transporting layer, light emitting layer, electron transporting layer, electron injecting layer is an EL layer.
In this specification, a light emitting element that is formed by a cathode, an EL layer, and an anode is referred to as an EL element. There are two kinds for forming the EL element; a simple matrix that an EL layer is sandwiched between two kinds of striped electrodes that run at right angles to one another, or an active matrix that an EL layer is sandwiched between a pixel electrode and a counter electrode arranged in matrix that are connected to a TFT. When the pixel density is become high, it is considered that an active matrix has an advantage over a simple matrix because the active matrix can drive at low voltage for having switches in each pixel (or each dot).
Since an EL material is deterioratable resulted from being oxidized or absorbed due to oxygen or moisture, there has been a problem that the luminous efficiency of a light emitting element is decreased or the lifetime thereof is shorted.
Conventionally, oxygen or moisture is prevented from penetrating into a light emitting element by encapsulating the light emitting element using an encapsulating can, enclosing a dry air thereinto, and pasting drying agent to the encapsulating can.
The conventional light emitting has the structure that has a light emitting element in which an electrode on a substrate is formed as an anode, an organic compound layer is formed on the anode, and a cathode is formed on the organic compound layer, and light generated in the organic compound layer is emitted through the anode formed as a transparent electrode to a TFT (hereafter, the structure is referred to as a bottom emission).
Although an encapsulating can is possible to cover a light emitting element in above bottom emission structure, the structure that an electrode on a substrate is formed as an anode, an organic compound layer is formed on the anode, and a cathode is formed as a transparent electrode (hereinafter, the structure is referred to as a top emission) cannot use the encapsulating can that is made from a light shielding material. A drying agent on the pixel portion disturbs the display in the top emission structure. Further in order not to absorb, the drying agent requires careful handling and quick enclosing.
Compared to a bottom emission structure, a top emission structure requires few material layers through which light is emitted generated in an organic compound layer, and thereby can suppress stray light between material layers having different reflective index.
An object of the present invention is to provide a light emitting device and a method for forming the light emitting device through which oxygen or moisture is prevented from penetrating into a light emitting element. Another object of the present invention is to encapsulate the light emitting element with a few steps without enclosing drying agent.
The present invention has a top surface emission structure in which a substrate, with light emitting elements formed thereupon, is bonded to a transparent sealing substrate. A pixel region is covered over its entire surface by a transparent second sealing material when bonding the two substrates, and is surrounded by a first sealing material (having a higher viscosity than the second sealing material) that contains a gap material (filler, fine particles, or the like) for maintaining a gap between the two substrates. The first sealing material and the second sealing material thus seal the light emitting element.
There is a fear in that air bubbles will remain in corners if a seal pattern shape for the first sealing material is formed into a square shape, an inverted “c” shape, or a “U” shape, and the two substrates are bonded by dripping the low viscosity second sealing material thereon.
Therefore, in the present invention, a pattern shape of the first sealing material is formed into a pattern having no bent portion (line shape) without making the pattern shape into the square shape, the inverted “c” shape or the “U” shape. Opening portions (four locations) are formed in the corners, which allow air bubbles to escape therethrough. By forming the opening portions, the low viscosity second sealing material is pushed out in the direction of the opening portion of the corners when bonding the two substrates using the low viscosity second sealing material. The two substrates can thus be sealed without air bubbles mixing in on the pixel region. In addition, a pattern for the high viscosity first sealing material may be slightly curved so that air bubbles do not form. Further, it is preferable that the substrate surfaces on the sealing side be smooth and have superior levelness so that bubbles do not mix in.
Further, there are cases in which a circumferential portion of the second sealing material will spread out from the opening portions (four locations), forming a bulging out shape (protruding shape), depending upon the viscosity of the second sealing material and the manner in which it is pushed out. There are also cases in which the circumferential portion of the second sealing material will form a shape that enters into the inside of the opening portions. Note that the adhesive strength between the two substrates can be increased in order to increase the contact adhesive surface area if there is provided a bulging out shape.
In either case, the high viscosity first sealing material functions to maintain the substrate gap through the gap material, and to adjust the planar shape of the low viscosity second sealing material. Further, the first sealing material can also serve as a mark when sectioning the substrate. For example, the substrate may be sectioned along the first sealing material when manufacturing a plurality of panels on one substrate, that is, in the case of so-called multiple patterns.
Further, a location of maximum load applied when a shock is received from the outside can be set to the location of the first sealing material (only the first sealing material has the gap material) disposed outside of the pixel region, and the load can be prevented from being applied to the pixel region. Further, this is a structure in which the first sealing materials are symmetrically disposed, and loads are applied uniformly and with a good balance. Shocks from the outside can therefore be uniformly diffused. Further, the first sealing materials have a symmetrical shape, and are disposed symmetrically, and therefore a very constant substrate gap can be maintained. That is, a light emitting device having an even more robust mechanical strength can be made by using the structure of the present invention.
Further, it is desirable that the substrate, through which light emitted from the light emitting elements passes, be thin for the top surface emission structure. Thin substrates have a disadvantage, however, in that they are weak with respect to shocks. Nonetheless, a light emitting device capable of withstanding shocks form the outside can be made in accordance with the present invention, even if a glass substrate or the like, which tends to break relatively easily, is used as the substrate through which light emitted from the light emitting elements passes. Further, there are no particular limitations placed on the transparent substrate used, and plastic substrates and the like can be used, for example. It is preferable that a pair of substrates use substrates having the same thermal expansion coefficient in order to maintain the adhesive strength.
Further, by making the seal pattern of the first sealing material into a simple shape, other seal pattern forming methods can also be used, such as a printing method, for example, in addition to a dispenser apparatus.
Further, the light emitting elements are sealed by the first sealing material, the second sealing material, and the substrates, and therefore moisture and oxygen can be effectively blocked. Note that it is desirable to perform bonding of the pair of substrates under a reduced pressure or in a nitrogen atmosphere.
According to an aspect of the invention disclosed in this specification, there is provided a light emitting device including a pixel portion having a plurality of light emitting elements between a pair of substrates, at least one of which has transmittivity, the light emitting elements each having:                a first electrode;        an organic compound layer on and in contact with the first electrode; and        a second electrode on and in contact with the organic compound layer;        
characterized in that:                the pair of substrates are fixed by a first sealing material disposed surrounding the pixel portion, and a second sealing material in contact with the first sealing material and covering the pixel portion; and        the first sealing material has openings in four corners.        
In the structure described above, it is characterized in that the first sealing material has a linear shape and is disposed in the plane of the substrate, in parallel with an x-direction or a y-direction.
Further, in the structure described above, it is characterized in that the second sealing material is exposed by the openings, and the circumference of the exposed second sealing material is curved, as shown by FIGS. 1A to 1C. A structure may also be adopted in which the second sealing material is exposed at the openings, and the circumference of the exposed second sealing material protrudes from the openings as shown in FIG. 1A. Alternatively, a structure may also be adopted in which the second sealing material is exposed at the openings, and the circumference of the exposed second sealing material is depressed inwardly from the opening portions, as shown in FIG. 1C.
Further, according to another aspect of the present invention, there is provided a light emitting device including a pixel portion having a plurality of light emitting elements between a pair of substrates, at least one of which has transmittivity, the light emitting elements each having:                a first electrode;        an organic compound layer on and in contact with the first electrode; and        a second electrode on and in contact with the organic compound layer;        
characterized in that:                a pair of first sealing materials sandwiching the pixel portion are disposed in an x-direction, and another pair of the first sealing materials are disposed in a y-direction;        a second sealing material fills a space between at least one pair of the first sealing materials; and        the shape of the second sealing material has bilateral symmetry, and is not limited to a linear shape provided that the second sealing material is disposed symmetrically sandwiching the pixel portion.        
Further, in each of the structures described above, the first sealing material contains a gap material that maintains a gap between the pair of substrates.
Further, in each of the structures described above, it is characterized in that the second sealing material has higher transparency than the first sealing material.
Further, in each of the structures described above, it is characterized in that the film thickness of the second electrode is from 1 nm to 10 nm.
Further, in each of the structures described above, it is characterized in that there is provided a protective layer having transparency and made from CaF2, MgF2, or BaF2, between the second electrode and the second sealing material.
Further, each of the structures described above is a light emitting device characterized in that light emitted from the light emitting elements is discharged through the second sealing material and one of the substrates.
Furthermore, the present invention can also be applied to double sided light emitting elements. In this case, each of the structures described above becomes a light emitting device characterized in that light emitted from the light emitting elements includes: emitted light that is discharged through the second sealing material and one of the substrates; and emitted light that is discharged through the other substrate.
Further, according to another aspect of the present invention in order to realize each of the structures described above, there is provided a method of manufacturing a light emitting device characterized by including the steps of:                forming one pair of first sealing materials on a first substrate in an x-direction, and one pair of the first sealing materials in a y-direction, opening a gap between each pair of the first sealing materials, for a total of four of the first sealing materials;        dripping a second sealing material having transparency onto a region surrounded by the first sealing materials;        spreading out the second sealing material so that it fills at least a space between mutually opposing first sealing materials when bonding the first substrate and a second substrate, upon which a pixel portion provided with light emitting elements is formed, so that the pixel portion is disposed in a region surrounded by the first sealing materials; and        curing the first sealing materials and the second sealing material.        
Further, according to another aspect of the present invention in order to realize the top surface structure of FIG. 1A, there is provided a method of manufacturing a light emitting device characterized by including the steps of:                forming one pair of first sealing materials on a first substrate in an x-direction, and one pair of the first sealing materials in a y-direction, opening a gap between each pair of the first sealing materials, for a total of four of the first sealing materials;        dripping a second sealing material having transparency onto a region surrounded by the first sealing materials;        spreading out the second sealing material so that it protrudes out from between adjacent first sealing materials when bonding the first substrate and a second substrate, upon which a pixel portion provided with light emitting elements is formed, so that the pixel portion is disposed in a region surrounded by the first sealing materials; and        curing the first sealing materials and the second sealing material.        
Further, a first sealing material and a second sealing material may also be formed on a second substrate, upon which a pixel portion is formed. According to another aspect of the present invention relating to a manufacturing method therefor, there is provided a method of manufacturing a semiconductor device characterized by including the steps of:                forming one pair of first sealing materials on a first substrate, upon which a pixel portion provided with light emitting elements is formed, in an x-direction, and one pair of the first sealing materials in a y-direction, opening a gap between each pair of the first sealing materials, for a total of four of the first sealing materials so as to surround the pixel portion;        dripping a second sealing material having transparency onto the pixel portion;        spreading out the second sealing material so that it fills at least a space between mutually opposing first sealing materials when bonding the first substrate and the second substrate; and        curing the first sealing materials and the second sealing material.        
In the structures relating to each of the manufacturing methods described above, the step of curing the first sealing materials and the second sealing material is a step of irradiating ultraviolet light or a step of heat treatment.
Further, in the structures relating to each of the manufacturing methods described above, it is characterized in that the second sealing material have a low viscosity than the first sealing material.
Further, in the structures relating to each of the manufacturing methods described above, it is characterized in that there is a further step of sectioning the first substrate and the second substrate along the first sealing materials after curing the first sealing materials and the second sealing material.
Further, although a metallic film having a thin film thickness and through which light passes, typically a film having aluminum as its main constituent, is used as the second electrode (cathode or anode) in the present invention, metallic thin films tend to easily oxidize, resulting in increasing their electrical resistance value. Further, there is a fear in that the second electrode will react with the constituents contained in the sealing materials. It is therefore desirable to cover the second electrode (cathode or anode), which is formed on a layer that contains organic compounds, by using a transparent protective film of CaF2, MgF2, or BaF2, for example, thus preventing reaction between the second electrode and the sealing materials, and also effectively blocking oxygen and moisture without using a drying agent. Further, it is possible to form CaF2, MgF2, and BaF2 by evaporation. Impurities can be prevented from mixing in, and the electrode surfaces can be prevented from being exposed to the ambient atmosphere, by forming the cathode and the transparent protective layer in succession by evaporation. Further, CaF2, MgF2, and BaF2 are stable materials as compared with LiF, do not diffuse to TFTs to exert almost no adverse influence.
Further, a region between the first electrode and the second electrode can maintain a non-oxygen state with a concentration as close to zero as possible, by using a metal (high work function material) having no oxygen atoms in its molecular structure, a tantalum nitride film, for example, as the first electrode, by using a metal (low work function material) having no oxygen atoms in its molecular structure, an aluminum thin film, for example, as the second electrode, and in addition, by covering these with CaF2, MgF2, or BaF2.
Further, although there are no particular limitations placed on the material used as the second sealing material, provided that it is a highly transparent material, it is desirable to use a material that blocks oxygen and moisture. Further, ultraviolet light is also irradiated to the pixel portion during curing if an ultraviolet curing resin is used as the second sealing material. It is therefore desirable to form a layer that absorbs or reflects only ultraviolet light, ZnO or the like, for example, on the transparent protective film.
According to another aspect of the invention disclosed in this specification, there is provided a light emitting device including a pixel portion having a plurality of light emitting elements between a pair of substrates, at least one of which has transmittivity, the light emitting elements each having:                a first electrode;        a layer containing an organic compound on and in contact with the first electrode; and        a second electrode on and in contact with the layer containing an organic compound;        a protective layer having transparency on the second electrode and made from CaF2, MgF2, or BaF2; and        a sealing material having transparency on the protective film;        characterized in that:        the first electrode is a laminate of metallic layers; and        the second electrode is made from a single layer of a metallic thin film having a film thickness of 1 nm to 10 nm.        
The metallic thin film uses aluminum as its main constituent in the structure described above. In the laminate of metallic layers, the layer that contacts the layer containing the organic compound is a layer made from titanium nitride. Further, the metallic layer may also be a single layer composed of titanium nitride, instead of a laminate.
Further, in the structure described above, it is characterized in that the first electrode contacts a source region or a drain region of a TFT, or the first electrode is electrically connected to the source region or the drain region of the TFT.
Further, in the structure described above the metallic thin film is on and contacting a layer made from CaF2, MgF2, or BaF2 that has a thinner film thickness than the metallic layer. In addition, there is a layer made from CaF2, MgF2, or BaF2 on and contacting the metallic thin film, and having a thicker film thickness than the metallic thin film. That is, the metallic thin film is sandwiched and protected by the layers made from CaF2, MgF2, or BaF2.
Further, in the structure described above, it is characterized in that the pair of substrates are fixed by a first sealing material disposed surrounding the pixel portion, and a second sealing materials that contacts the first sealing material and covers the pixel portion and the first sealing material has openings in its four corners.
Note that the light emitting elements (EL elements) have a layer (hereinafter referred to as an EL layer) that contains an organic compound, in which luminescence (electroluminescence) developing by adding an electric field is obtained, an anode, and a cathode. Light emission when returning to a base state from a singlet excitation state (fluorescence) and light emission when returning to a base state from a triplet excitation state (phosphorescence) exist as types of organic compound luminescence. Light emitting devices manufactured in accordance with the present invention can be applied to the use of either type of light emission.
The light emitting elements having the EL layer (EL elements) have a structure in which the EL layer is sandwiched between the pair of electrodes, and the EL layer normally has a laminate structure. The laminate structure of a hole transporting layer, a light emitting layer, and an electron transporting layer, that has been proposed by Tang et al. of Eastman Kodak Company can be given as a typical example. This structure has extremely high light emission efficiency, and nearly all light emitting devices undergoing research and development at present employ this structure.
Further, a structure in which a hole injecting layer, a hole transporting layer, a light emitting layer, and an electron transporting layer are laminated in order on an anode may also be used. A structure in which a hole injecting layer, a hole transporting layer, a light emitting layer, an electron transporting layer, and an electron injecting layer are laminated in order on an anode may also be used. A fluorescent pigment or the like may also be doped into the light emitting layer. Further, these layers may be formed by using all low molecular weight materials, and may also be formed by using all polymeric materials. Further, layers that contain inorganic materials may also be used. Note that all of the layers formed between the cathode and the anode are referred to generically as EL layers in this specification. Hole injecting layers, hole transporting layers, light emitting layers, electron transporting layers, and electron injecting layers are therefore all included in the category of EL layers.
Further, there are no particular limitations placed on a method of driving a screen display in the light emitting device of the present invention. For example, a dot sequential driving method, a line sequential driving method, a surface sequential driving method, or the like may be used. A line sequential driving method is typically used, and a time divided gray scale driving method or a surface area gray scale driving method may also be appropriately employed. Further, image signals input to a source line of the light emitting device may be analog signals and digital signals. Driving circuits and the like may be appropriately designed according to the image signals used.